A couple of days ago, we headed out on an evening hike. The temperature was just below zero and there wasn’t a covering of snow yet. On route we saw these magnificent ice formations. There was a ditch running alongside the trail, and the ice was protruding out of the soil on the far slope.
I found it is most often called needle ice, although some of its other names are more poetic such as ice castles, frost castles, crystal castles, and frost pillars.
This type of ice only forms under certain conditions. First, there needs to be ground moisture. Second, the soil has to have the right porosity to allow capillary action to occur. Thirdly, the temperature of the soil has to be above freezing, while the temperature of the air above has to be below freezing.
As a little science refresher, capillary action occurs when the diameter of a tube is small enough that the forces of adhesion (between the liquid and the edge of the tube) and cohesion (between the molecules of water) cause the water to climb upward, rather than sink downward with gravity.
So, the groundwater in soils, with the right-sized spaces, wicks upward or outward by capillary action and then freezes when it meets the below-freezing air. This continues, the ice grows up or out, and can lift or push out small soil particles that can be seen on the top of the needles.
It is likely you have walked over needle ice and know it by the satisfying crunch underfoot. Sometimes you need to get down and brush off some leaf litter to get a good look!
So beautiful! Kate
Here is a video to help explain capillary rise in soils:
And gratitude to our little night-hiking crew (good company and good eyes):
